Process of plating palladium and plating bath therefor



Patented Oct. 26, 1948 STATES George C. Lambros, New York, N. Y.

No Drawing. Application February 28, 1946, I 'Serial'No. 651,041

Claims. ('01. 204-47) This application is a continuation-in-part of my application Serial No. 443,222, filed May 16, 1942, which has been abandoned, and this invention relates to a process of electrodepositin'g metals of the platinum group, particularly palladium, a plating bath for use in carrying out said process, and a method of preparing said bath.

Until recently there has been practically no industrial or commercial application for electrodeposits'of pure palladium. However the present plentiful supply of the metal resulting from rapid developments in the extraction of nickel from sulfidic ores, and the relatively low price of palladium as compared With platinum and gold, coupled with the fact that a given weight of palladium will plate about twice as much surface area as will the same weight of gold or platinum, have created a substantial demand for palladium plating.

Difliculty has been encountered in producing satisfactory methods and baths for electrodepositing palladium. Several attempts have been been made including the proposal to use a bath formed by dissolving a crystallized amino-cyanide of palladium in water and addingammonia or sodium hydroxide, but the known methods and baths have serious disadvantages or short-comings, such as instability or rapid deterioration of the bath, inefiiciency in the plating operation and lack of coherency in the deposit.

Therefore, prime objects of my invention are to provide a method of electroplating palladium and a bath therefor whichgshall embody novel and improved features whereby the bath shall be substantially more stable than known .palla-. dium plating baths and the plating operation can be carried on for exceptionally long periods of time without series deterioration of the bath; and to provide a simple, efiicient, and relatively inexpensive method and bath for electrolytically producing exceptionally coherent deposits of palladium with either brilliant or matte surfaces.

Other objects, advantages and results of the invention will be brought out by thefollowing description.

I have discovered that the above-mentioned disadvantages of the prior methods and baths may be overcome, and that the foregoing objects may be attained, by electroplating from a bath prepared by dissolving dichlorodiamine palladium [Pd (NHs)2 C12] in a solution of sodium or potassium cyanide.

More particularly I have found that best results can be obtained when the weight of sodium or potassium cyanide bears a specific ratio ously affects the'bath. However, it is my belief ladium compound occurs.

2 to the weight of palladium metal in the form of dichlorodiamine salt.

Furthermore, it is desirable that the dichlorodiamine palladium be freshly prepared immediately before the addition of the sodium or potassium cyanide.

' In preparing the bath, the premilinary steps may closely follow the method advocated by the United States Bureau of Standards for the purification of palladium metal. More specifically stated, a weighed sample of palladium is dissolved in nitric acid and the solution is evaporated to a syrupy consisteny in a covered porcelain dish. The syrupy residue is dissolved in water by a little hydrochloric acid, and the solution is diluted to 40 to 50 grams of palladium per liter. Ammonium hydroxide is then added until no further precipitation of the flesh-colored pal- The solution is then heated on a steam bath for 15 ,to 20 minutes to redissolve the precipitate, whereafter a little more ammonium hydroxide is added with care to avoid excess, and the solution is allowed to cool. Hydrochloric acid is then added until no more precipitation of the yellow salt occurs, but an excess of hydrochloric acid should be avoided.

To this freshly prepared solution, C. P. sov which of course produces an excess of cyanide within definite limits. If less than the above stated minimum is used, the precipitate will not completely dissolve, while if more than the stated maximum is added, the bath will not operate sat isfactorily.

, This ratio may be expressed in terms of weight of cyanide (UN) to weight of-palladium metal as follows: from 0.63 gram to 0.96 gram of cyanide (CN) for every gram of palladium metal.

No attempt is made to explain the chemical.-

compounds that are formed upon the addition of thecyanide, or why an excess of cyanide deleterithat my bath has free chloride ions (01) and that these in large measure account for the stability and long life of the bath.

The solution is made up in concentrated form of about forty (40) grams of palladium metal per liter, and thereafter diluted with water to make the bath. This concentrated solution is also used as a replenisher and is added to the bath from at a bath potential of. from 1 to 2 volts. Platinum 1 or palladium anodes are preferred. The palladium metal content of the bath is maintained within the range of 0.5 gram to grams, preferably from 1 to 5 grams palladium, per' liteniby periodic. additions of the concentrated solution of the (iichlorodiamine palladium and cyanide. k In preparation of the concentrated solution,

have found that the stability ofthhsolutiOnxis:

improved and more of the palladium is made available in plating form, by heating the mixture for about four hours at a etemperature of from about 60 C. to about 95 C.",and the improvement is substantiallygreaterwheman'amounflof hyt droxide of sodium or potassium substantially.

equal to the amount 'of potassium cyanide or sodi um cyanide, is added-with the cyanide to the freshlyiarepared dichlorodiaminepalladium saltr' I he bath remains exceptionally stabl'e'andhas been-*fo'undto operate'ior many Weeks, *more' or less constantlyand uniformly satisfactorily, with only periodic additions of the concentrated-solutionriThis is remarkablebecause the quantity of cyanide would'b'eexpected to be reduced, es-" peciallywith the bath, operating at close toboiling temperatures.

The color or composition of the palladium'deposit may be varied to suit the taste;purpose or r use of the plated articles, bythe addition to the bath of small amounts of metals-such'as tim' cobalt; nickel; zinc,=copper, gold or indium; The

addition of sodium stannitei'ior example'rto the palladiumcyanide'bath in small quantities until the desired effect is obtained, results, in adeposit comprising both palladium and tin. Preferably" the second metal is added to the bath in the" iorm of an alkaline'solution of'that metaland in small quantities until thede'sired change in the'palladi um plate is effected. Brighter matte surfaces" and plating of various colors can be produced in this'simple manner. v

Similar bathsio'r electrodepositing 'other 'metals of the platinum group maybeprepared-by dis solving in awatersolution of sodium or potassium cyanide, 'salts of those metals that' willform-am ino-cyanides of 'the""-platinum* metals-in such solutions:

Although the' present invention-has been described in connection with certain preferred em bodinients' thereof, "it is to be understood-- that variations and modiflcations may ber'esortedto which will be within the spirit and scope attire-in:-

vention'. I

Hai ing thus described tlie'invention, what I claim is: Y

1. A process-of electrodepositing palladium'x:

whichcomprises dissolving freshly prepared dichlorodiamine palladiumin an aqueous solution of a cyanide "of the group consisting of sodium Meta'lindustry mewYork),'vol. 26,*No. fifpage cyanide and potassium cyanide, said solution containing cyanide in an amount ranging from 0.63 to 0.96 gram of cyanide (CN) per gram of palladium in the form of dichlorodiamine palladium, and from 0.5 to 15 grams of palladium metal per liter, and electrolyzing said solution in the presence of said cyanide,

2. A .process cor Lelectrodepositing palladium which comprises: dissolving tfreshly precipitated dichlorodiamine palladium in an aqueous solution of cyanide of the group consisting of sodium cyanide and potassium cyanide, said solution containin'g cyanide in an amount ranging from 0.63 to 0;69 gram=of cyanide (CN) per gram of palladium inthe rormiofrdichlorodiamine palladium, and from 0.5 to 15'grams of palladium metal per liter, and 'electrolysing-said solution in the presence of said cyanide at a temperature of about C., and replenishingithe solution periodically to maintain the palladium content of the solution from 0.5 to 15 grams of metal per liter.

31A bath fortthe electrodepositio'n of palladium um in theiform of dichl'orodiamine palladium, and from 0.5 to 15 rams. of palladiummetal per liter.

4. The method '..of preparing a concentrated solution for use in preparing baths for electro deposition'of palladium comprising the steps of dissolving freshly "precipitated dichlorocliamin'e palladium in a solution ofxa cyanide of the group consistingof 's'odium'cyanide' and potassium cya-l. nide, said solution containing about. forty grams of palladium metal per liter and from 053 gram to 0.96 gram'of cyanide for each gram. of palladie;

um, and heating said solution for about fourhours at a temp'eratureof from about 60C. to about C.

5,.The method as defined in claim 4.with the additional stepof adding to the mixturedurin'g heatingltheifeof' an'amountof sodium or potassiumrhydroxide substantially. equal to: the amount of cyanide;

GEORGElC. LAMBROS.

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